The present invention relates to multi-beam satellite communications and more specifically to a multi-beam satellite communications payload configuration that allows for efficient and flexible allocation of power to the different high power amplifiers to meet varying traffic conditions or variable link conditions.
The requirement of flexible power allocation is of interest to communications satellite operators for maximizing the overall satellite throughput capabilities and matching the evolution of traffic through the satellite life with the associated increase in revenues.
A common technique for providing flexible power allocation to beams depending on traffic demands consists in using multiport amplifiers in antenna configurations using either a single feed per beam or in focused multi-beam reconfigurable reflector antennas with several feeds contributing to each beam.
The main characteristic of a multi-port amplifier is that the signals corresponding to each input port are first split by an input matrix in such a way that all beam signals are amplified by all the high power amplifiers (HPAs). Therefore, and independently of the beam power distribution, equal loading of all the HPAs is achieved, maximizing the overall DC to RF efficiency. Depending on the beam input considered, the input matrix generates different relative phases at the HPA inputs. Then the output matrix is able to separate again the signals that belong to each input port to the corresponding output port.
One of the main problems associated with this technique is the need of keeping a good phase and amplitude tracking between the different HPAs in order not to degrade the beam isolation performance. This problem becomes more severe as the frequency of operation increases and it is quite serious at Ka band. As a consequence, phase and amplitude control elements are required to correct for the phase and amplitude tracking errors due to temperature and ageing effects.
Another associated problem is the need to implement adequate redundancy units to cope with the eventuality of HPA failures in order to maintain the required total RF power. Therefore, a complex redundancy scheme is required to substitute the failed HPAs by the cold redundant ones. A large number of RF switches are to be implemented with significant impact on output losses, mass, accommodation, reliability and cost. In addition, a multi-port amplifier structure needs to be readjusted in terms of phase and amplitude tracking after the redundancy configuration has been changed, in order to guarantee good RF beam isolation.
Multiport amplifiers have been successfully implemented at L and S bands in different missions. However, the implementation of multi-port amplifier concept to higher frequencies (Ku and Ka bands) is not easy due to the magnification of the problems mentioned before. It is well known that the impact of the above mentioned phase and amplitude tracking errors is alleviated by the use of large order multi-port amplifiers (i.e. 16×16 structures). Nevertheless, cold redundant HPA units and complex reconfiguration switch matrices are still required to keep power and beam isolation within acceptable limits.
U.S. Pat. No. 5,818,388 discloses an apparatus that provides active redundancy, without using redundancy switches, for high power satellite communications payloads. This apparatus uses a number of power amplifiers equal or greater to the number of antenna feed elements to provide extra power at the beginning of life. As power amplifiers fail, the amplitude and phase of the signals driving the remaining power amplifiers are adjusted to maintain antenna performance. In this apparatus, each of the plurality of power amplifiers has a sufficient excess output power capacity at the beginning-of-life point to ensure that, given an anticipated maximum number of power amplifiers failures, performance requirements can be met at an end-of-life point.
U.S. Pat. No. 6,091,934 discloses another technique to ensure dynamic allocation of power to satellite's high power amplifiers to maintain amplifier efficiency and meet peak traffic demands and reduce power consumption during low traffic periods. This is accomplished by monitoring traffic on the channels and allocating available power based on the traffic. Command signals received from a ground station are used to direct the power allocation system to the power amplifiers and adjust the RF power level.
It is also known that the output power from a travelling wave tube amplifier (TWTA) can be adjusted by changing its bias conditions. Recent measurements have proven the possibility of controlling the TWTA saturated power with almost negligible degradation in efficiency. At Ku and Ka bands, 3 dB output power control has been measured with very little (around 2 percentage points) efficiency degradation. This technique is very simple in concept and very well performing. The drawback is the limitation to the case of single feed per beam antenna architectures. Other antenna architectures using overlapping clusters of a few feed per beam are not easily adapted to the use of the flexible TWTA concept.